Electro-optical modulator module for CO2 laser Q-switching, mode-locking, and cavity dumping

ABSTRACT

Various electro-optical modulator module designs are presented, which can provide for uniform, symmetric, and efficient heat removal for mode-locking, Q-switching, and/or cavity dumping operations. Heat can be uniformly extracted from an EO crystal without imposing undue stress, thereby preventing birefringence and laser beam degradation. A liquid-cooling approach can be used for high-duty operations, such as mode-locking operations. Efficient heat removal can prevent thermal run-away from electrical heating of the crystal due to the large drop in the electrical resistance of CdTe with increasing temperature when operated above 50° C. RF or video arcing and subsequent damage to the EO crystal can be prevented by surrounding the crystal with a low dielectric constant material that lowers the capacitance coupling to ground, while still maintaining good thermal cooling.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the performance of optical qualityelectro-optical crystals used in or with lasers and laser systems.

BACKGROUND

As laser system technology improves and laser pulses continue toshorten, drilled holes, cuts, and scribed grooves that are formed onvarious materials by these laser systems continues to improveaccordingly. Improvements are observed, for example, in recast surfaceresidues around the edges of these features, and in the smaller taper ofthe features in the material being processed. Reduction and/orelimination of micro-cracks around these features also is observed.

For the same amount of average laser power delivered to a work-piece,the cost of a laser system increases dramatically as the pulse durationdecreases. The cost of a CO₂ laser is generally much lower than the costof a diode pumped solid state (DPSS) laser, while the operating lifetime, size, weight, and reliability are comparable. For a given pulsewidth available from a CO₂ or DPSS laser, then, the wavelength of thelaser becomes the discriminating parameter (in addition to costconsiderations) when attempting to obtain the most favorable holes,cuts, or grooves in the material to be processed. For example, thecharacteristics of these features can determine whether a mid IR (i.e.,CO₂ at 9.2 to 10.6 microns), near IR (i.e., DPSS at around 1 to 1.5microns), visible (second harmonic of DPSS lasers), or UV (excimer or3^(rd) harmonic of DPSS lasers) is selected for performing the process.Short CO₂ pulses also are of interest to the scientific community toprobe the atomic and molecular relaxation rates.

Presently, the primary techniques used to obtain short laser pulses froma laser system include Q-switching, simultaneous Q-switching andCavity-dumping, and mode-locking. Each of these short pulse generationtechniques requires one or more electro-optical switches, orelectro-optical modulators, to be inserted within the feedback cavity ofthe laser system. A cadmium tellurium (CdTe) crystal is presently theelectro-optical crystal of choice for CO₂ laser systems. Performingthese short pulse generation techniques in CO₂ lasers with CdTecrystals, however, presents problems that need to be addressed in orderto maximize performance.

For example, the drive voltage for electro-optical and acousto-opticalswitches (or modulators) is proportional to the laser wavelength.Consequently, the modulators for CO₂ lasers (i.e., operating in the 9.2to 11 micron region) require approximately 10 times more voltage thanfor lasers operating, for example, in the 1 micron range. The highvoltage requirement complicates the design of the electro-opticalcrystal holder for CO₂ laser mode-locking and Q-switching applications,as it is necessary to prevent arcing and/or dielectric breakdown of theelectro-optical crystal by either the high video voltage for Q-switchingand/or cavity dumping, or by the RF voltage for mode-lockingapplications.

Further, the optical absorption of existing acousto-optical devices istoo large to be inserted into a CO₂ laser feedback cavity and stillobtain reasonable laser efficiency. Consequently, CdTe electro-opticalcrystals are the present material of choice because these crystals havelower optical absorption in the 9.2 to 11 micron range thanacousto-optical devices that presently use Ge as the acousto-opticmedium. CdTe crystals have relatively poor thermal conductivity,however, and uniformly extracting the heat without imposing stress andcausing birefringence is challenging. Further, non-uniform heatextraction can lead to spatial variations of the refractive index, whichcan produce an undesired bending or deflection of the laser beam.

Anti-reflection films are presently required to be deposited on theentrance and exiting surfaces of the CdTe crystal in order to reduce theoptical loss within the laser cavity. These films have a low opticaldamage threshold, such that obtaining high laser reliability under highlaser peak power and high laser pulse energy operation required for mostmaterial processing applications is difficult.

Other potential problems must be considered when addressing the heatremoval from a CdTe crystal assembly. For example, the electricalresistance of CdTe crystals drops dramatically with increasingtemperature, thus increasing the difficulty of controlling thetemperature of the crystals. It is not uncommon for the electricalresistance to drop from 30 to 50 times the room temperature value whenthe crystal is operated above 50C. Fortunately, if the crystal does notexceed 100C, such as due to effective heat removal, the thermalresistance recovers when the crystal is returned to room temperature.Further, a CdTe crystal can easily suffer damage from RF arcing from anymetal parts in the housing that are too close to the crystal, even whensuch metal parts are separated from the crystal by a dielectric becauseof the increased capacitive coupling caused by the dielectric constantof the dielectric.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view diagram of an electro-optical(EO) modulator module assembly that can be used in accordance with oneembodiment of the present invention.

FIG. 2 is an end cross-section section view of the module assembly ofFIG. 1, looking down the length of the crystal.

FIG. 3 is a cross-sectional view of the module assembly of FIG. 1,looking down on the module from the top in FIG. 1.

FIG. 4 is an external side view of the EO module assembly of FIG. 1.

FIG. 5 is a schematic diagram showing a folded waveguide laser systemthat can be used with the module assembly of FIG. 1.

FIG. 6 is a plot showing the operation of a FM mode-locked CO₂ laser asper the configuration of FIG. 5 with the pulsed RF power applied to thedischarge of the CO₂ laser.

FIG. 7 is plot showing the individual CO₂ laser mode-locked pulses ofFIG. 6.

FIG. 8 is an exploded perspective view diagram of an electro-optical(EO) modulator module assembly that can be used in accordance with asecond embodiment of the present invention.

FIG. 9 is (a) a side cross-sectional view, (b) a first endcross-sectional view, and (c) a second end cross-sectional view of themodule assembly of FIG. 8.

FIG. 10 is a plot showing the power output of a Q-switched waveguide CO₂laser as a function of time, utilizing a modulator design of the priorart.

FIG. 11 is a plot showing the power output of laser of FIG. 10,utilizing a modulator design as in FIGS. 8-9.

DETAILED DESCRIPTION

Systems and methods in accordance with embodiments of the presentinvention can overcome various deficiencies in existing laser systems,including those described above. These embodiments can utilize animproved electro-optical (EO) modulator module including an activeelectro-optical crystal of a material such as CdTe. Such a module can beused for either Frequency Modulator (FM) or Amplitude Modulator (AM)mode-locking, or for Q-switching/cavity dumping, by properly orientingthe CdTe crystal axis with respect to the applied electric fieldgenerated by the electrodes, and with the polarization of the laserradiation propagating down the length of the crystal as known to one ofordinary skill in the art.

In applications involving Q-switching or cavity-dumping operation,acceptable operation can be obtained with air cooling. Atmospheric aircooling can be sufficient for these operations due to the low duty cyclerequirements, instead of the high or continuous duty cycle used for mostmode-locking operation. If lower laser mode-locking duty cycle operationcan be accommodated, air-cooling can suffice as in Q-switching or cavitydumping applications.

For many mode-locking operations, however, air cooling is notsufficient. For these operations, it is necessary to provide improvedcooling and/or heat removal that does not impart undesirable stress onthe crystal or produce other undesirable effects. A design that canprovide such improved cooling is shown in the embodiment of FIGS. 1-4,which carry over reference numbers for simplicity. FIG. 1 shows anexploded perspective view of an exemplary EO modulator module assembly100 that can be used in accordance with one embodiment of the presentinvention. These components can be contained within an appropriatemodulator housing (138 in FIGS. 3-4), such as a metal housing, toprovide RF shielding for mode-locking operation. A portion of a metalhousing can include a plastic plate (122 in FIGS. 3-4), fabricated froma material such as Noryl®, available from Boedeker Plastics, Inc. ofShiner, Tex., a polyphenylene oxide—styrene alloy, such that anycompression springs or other fastening devices passed through the plateto close the housing, as well as to apply pressure to hold theelectrodes against the crystal as described below, can prevent arcingand corona discharges that can otherwise form in the module.

The module can contain an elongated, rectangular CdTe crystal 102. Inorder to obtain optimum performance from the crystal, it is desirable toapply no more pressure to the crystal than is necessary to hold thecrystal in place. The components used to hold the crystal can bearranged to allow for a normal expansion and contraction of the crystalwithout causing excessive stress. To obtain optimum performance, thecrystal also can be uniformly and/or symmetrically cooled, such as by aflow of liquid or gas, such that the temperature stays below about 70°C. Maintaining the temperature of the crystal can prevent a substantialchange in thermal resistance. Further, minimizing temperature gradientsacross the crystal that would arise from uneven cooling can help toavoid significant bending and lensing of the laser beam propagating downthe length of the crystal due to refractive index gradients.

The exemplary assembly of FIGS. 1-4 can be used to obtain sufficientcooling without placing undue stress on the crystal. Metal electrodes112 can be placed on either side of the crystal 102. Soft metal cushions110 can be placed between the crystal 102 and the electrodes 112 inorder to ensure good thermal and electrical contact between theelectrodes and the crystal, without placing undue stress on the crystal.The metal cushions can be made of any appropriate metal of sufficientsoftness and conductivity, such as for example Indium. A conductor, suchas a flexible copper ribbon 134, can serve as the RF connector to theelectrodes for a mode-locking modulator, or as a high video voltageconnector to the electrodes for a Q-switch or cavity dumped modulator.The electrodes 112 each can be pressed against the metal cushions 110 bya dielectric wall structure 114. The dielectric wall structures caninclude any of a number of fastening, clamping, or holding mechanismsfor applying pressure to the electrodes. In one embodiment, six screwassemblies 136 distributed symmetrically about at least one of the wallstructures are used, with each screw assembly including a plastic tipembedded in holes drilled in the dielectric wall structure 114.

The dielectric material of the wall structures can have a good thermalconductivity and a low dielectric constant, in order to reduce the RFcapacitance loading of the structure. A material that has producedfavorable results as the wall structure is a Beryllium Oxide (BeO)ceramic material. Surrounding the crystal with a low dielectric constantmaterial that lowers the capacitance coupling to ground can prevent RFand/or video arcing and subsequent damage to the EO crystal. Thedielectric wall structure can be machined with a rectangular slot in thecenter, through which a flexible copper conductor 134 can be passed.Cooling passages 128 can be machined within the wall structures to allowliquid coolant to flow the length of the structure. The position of thepassages 128 can be machined symmetrically with respect to the crystal(such as shown in FIG. 2), in order to provide for symmetric heatremoval. The passages can be machined over the entire length of the wallstructures for simplicity, and can be capped or plugged at each end byan appropriate plug 130, such that the cooling liquid does not leak intothe module. The liquid can be directed into, and out of, the coolingpassages 128 using fluid channels 118, such as channels of plastictubing that are press-fitted onto barb fittings 116 epoxied to thedielectric wall structures 114. The coolant can flow from a coolantreservoir (not shown), through one of the channels 118, through theentrance and exit of the cooling passages 128, then back through achannel 118 to the reservoir. As shown in the Figure, there can be twoparallel cooling passages in each wall structure. In order to reduce themagnitude of heat gradients across the wall structure, the coolant canbe directed to flow in opposing directions in the passages (as bothflowing from right to left in the Figure could result in a highertemperature at the left of the wall structure). Such liquid cooling wasfound to provide acceptable operation with continuous and high dutycycle RF excitation of the crystal that can be required for mode-lockingthe laser.

The heat removal and performance of the module can depend at least inpart upon the coolant chosen to flow through the cooling passages 128.The electrical resistivity of a typical water-based coolant liquidhaving a corrosive inhibitor additive to prevent corrosion in Aluminumcooling coils (such as DOWFROST* HD available from the Dow ChemicalCompany of Midland, Mich.) can be too low for use in an RF modulatormodule. The low RF resistance of such coolants can lead to undesirableheating of the coolant, and a subsequent heating of the modulator moduleitself. The low RF resistance also can make it difficult to obtain thehigh RF voltage needed to obtain good mode locking performance. In oneembodiment, a preferable coolant was found to include a mixture of 10%corrosion inhibitor such as Optishield® II, sold by OptiTemp of TraverseCity, Mich., and approximately 90% de-ionized water. This mixturesatisfies the requirements for corrosion prevention, cooling, and highRF resistance. In a system without an aluminum cooling coil, such as asystem that utilizes copper for the cooling system, de-ionized water canbe an acceptable solution as corrosion will be prevented and a high RFresistance maintained.

In order to provide support orthogonal to that of the electrodes 112, apair of ceramic holders 104, 106 can be used to contact the sides of thecrystal 102 that are not contacted by the soft cushions 110. Theseceramic holders can be made of any appropriate thermally conductingmaterial, such as a BeO ceramic, and can be machined for uniform contactbetween the holders and the EO crystal 102, thereby obtaining goodthermal conductivity from the crystal to at least one of the wallstructures 114 in contact with the holders. At least one of the ceramicholders can include a lower holder 106 in combination with an upperwedge portion 108. As shown in the end view cross-section of FIG. 2, thetop-view cross-section of FIG. 3, and the side view of FIG. 4 showingthe assembled module components in the module housing 138, screwassemblies 136 can be used with wall structure 114 to move a slidingmember 132 such as a plastic end cap, such that the end cap pressesagainst the BeO wedge 108. Tightening the screw assembly pushes the endcap against wedge 108, which then is forced to slide (to the right inthe Figure) on the lower BeO wedge 106. This arrangement provides anadjustable downward force on the EO crystal 102, while also providing agood conductive cooling path for the EO crystal. An adjustable amount ofdownward force between about 2 lbs. and about 10 lbs. has been found toserve the purpose of holding the crystal in one embodiment, whileproviding the required thermal conduction and pressure adjustment toobtain uniform stress on the crystal. Such an assembly also allows allof the pressure adjustment screw assemblies to be positioned along oneof the wall structures 114, simplifying both operation and design. Forexample, additional screw assemblies can be passed through the wallstructure to apply pressure to metal electrode 112 in order to ensuregood and uniform thermal and electrical contact with the crystal, aswell as to press against the BeO ceramic holder 104 upon which the CdTecrystal 102 rests. The surfaces of the BeO ceramic holders 104, 106 andthe sliding surfaces between BeO parts 108 and 106 can be fine machinedfor substantially uniform contact, as well as to place substantiallyuniform pressure on the EO crystal 102 while holding the crystal inplace. The uniform contact also helps to obtain good thermalconductivity from the crystal 102 to the BeO wall structure 114. Thescrew assemblies can be adjusted such that a minimum sufficient and/oruniform stress is placed on the EO crystal. The screw assemblies can beadjusted manually or using mechanical means, such as in combination witha feedback loop, in order to apply an even and sufficient amount ofpressure. Applying a minimum and uniform amount of pressure to thecrystal on each of the four sides of the elongated rectangular crystalcan help to minimize birefringence and bending of the laser beam, aswell as to obtain good thermal contact such that heat flows evenly fromthe crystal.

As shown in FIG. 1, the module assembly can include window holders 124on each end of the assembly. These window holders in one embodiment arepolyetherimide plastic parts, which are 30% glass filled. Glass-filledpolyetherimide has a relatively high strength that allows the materialto easily be cut to form, in order to properly to support the windows,while also not being electrically conductive. The glass filling alsoallows threads to be machined into the window holders such that bolts orscrew assemblies can be used to connect the window holders to thehousing 138. Each window holder 124 can hold a window 126 for themodule, with each window being held against an end of the crystal 102.These windows can be made of any appropriate material, but in oneembodiment comprise zinc selenide (ZnSe) windows. As explained in U.S.Pat. No. 5,680,412, issued on Oct. 21, 1997 and hereby incorporatedherein by reference, ZnSe windows can improve the optical damagethreshold of a CdTe crystal. The ZnSe window holders 124 in such anembodiment can be fabricated from a plastic dielectric such as Ultem2300. Another window holder, or a panel or other mechanism (not shown)can be used to hold the window against each window holder 124. Theopenings in the window holders through which the laser beam passes canbe larger than the beam diameter, in order to avoid burning the edges ofthe opening. If an outer window holder is used, cross-cuts can be usedto provide a flexible spring-like and firm force to hold the windowsagainst the end of the EO crystal.

In the embodiment shown in FIGS. 1-4, superior cooling of the EO crystalcan be obtained by passing heat through the copper electrodes 112 to thetwo BeO wall structures 114, which include the cooling passages 128allowing most of the heat to be carried away by the coolant. Thermalcooling of the EO crystal also occurs through the top and bottomsurfaces of the EO crystal by thermal conduction through the BeO slidingpieces 106, 108 and the stationary piece 104 into the BeO wall structureagainst which the pieces are being pressed, such that the heat also canbe taken away by the flowing coolant. This symmetric cooling of the EOcrystal can result in minimum distortion of the transiting laser beamdue to deflection or lensing effects caused by refractive indexvariations arising during the warm-up/cooling transients.

Heat also can be transmitted to the metal housing 138 from BeO pieces104 and 108, as well as from the wall structures 114. Sufficient coolingwithout fluid flow can be obtained for those applications where the dutycycle is low and, consequently, there is less heat removal required.Some small beam distortion can occur during the warm up time of themodule since the cooling in vertical and horizontal axes of the crystal,for example, is not uniform without liquid cooling. The laser resonatorcan be re-adjusted after the module has reached thermal steady statecondition in order to compensate for the distortion.

Such a modulator module can be used with either FM or AM modulation tomode lock a laser by properly orienting the crystal axis. Axisorientation is well known in the art, such as described, for example, inthe reference by A. Yariv, Quantum Electronics, 3^(rd) edition: JohnWiley & Sons, 1985. It also is well known that FM mode-locking requiresless voltage than AM modulation and is therefore preferred for manyapplications, such as is shown in the reference by D. J. Kuizenga and A.E. Siegman, “FM and AM Mode-Locking of the Homogeneous Laser-Part ITheory,” IEEE J. Quant. Electron; QE-6, November 1970, p. 694. ForQ-switching CO₂ lasers, the CdTe crystal axes with respect to thepolarization of the laser can be arranged as for AM mode locking in themodulator module design of FIGS. 1-4. It also is well known that shortermode-locked CO₂ laser pulses can be obtain by increasing the pressure ofthe gas mixture within the laser head, thereby increasing the laser gainbandwidth. See for example the reference by Knut Stenersen, Stig Landro,Per Inge Jensen, and Stian Lovold, “FM Mode Locked High Pressure CW RFExcited CO ₂ Waveguide Laser,” IEEE J. of Quant. Elect., Vol. 27, No. 7,July 1991 pp. 1869-1873.

It can be advantageous for many applications to keep the pulserepetition rate of a mode-locked laser relatively low. Unfortunately, alow mode-locked pulse repetition frequency can necessitate a longerlaser resonant cavity, which can increase the difficulty in aligning,and maintaining the alignment of, the Fabry-Perot cavity. Fortunately, afolded “saw-tooth” waveguide laser design can be used as shown in FIG.5(a). Examples of waveguide laser designs are described in U.S. Pat. No.6,192,061 B1, issued Feb. 20, 2001, entitled “RF Excited WaveguideLaser;” and U.S. Pat. No. 6,697,408 B2, issued Feb. 24, 2004, entitled“Q-switched Cavity Dumped CO₂ Laser for Material Processing,” each ofwhich are hereby incorporated herein by reference. Such a design canprovide a compact, relatively low pulse repetition frequency mode-lockedCO₂ laser package with reasonable average power output that is suitablefor industrial/scientific applications. For example, a 5-folded, squarebore waveguide 200 as shown in FIG. 5(a) can be configured with near100% reflectance mirrors 202 to set the direction and length of eachpass through the waveguide. The total cavity length is approximately 265cm in an embodiment where each pass though the waveguide isapproximately 53 cm in length. A partially reflecting output mirror,with a reflectance on the order of about 70%, can be used at the outputof the waveguide. Another near 100% reflectance window 206 can be placedon the other side of the modulator module 208, in order to serve as aresonator mirror for the beam path through the crystal. The orientationof the crystal in the modulator module in FIG. 5(a) is shown in FIG.5(b), with the electrodes 210 placed at the top and bottom of thecrystal 212 in the Figure, so as to apply a vertical electric fieldacross the crystal. The crystal in this embodiment is 5 mm by 5 mm incross section, and about 50 mm in length. Such a design can yield amode-locked pulse repetition frequency of approximately 56.6 MHz, withpulse widths of approximately 3.2 nanoseconds at an operating pressureof around 100 Torr with approximately 23 W of output power. A switch 214can be used with the waveguide in order to super-pulse the laserdischarge, such as by using about a 100 MHz discharge drive voltage andelectrodes on either side of the waveguide. FM modulation was used withthe EO modulator module in experiments to obtain these results, with theRF drive power to the laser discharge being pulsed at approximately ½the duty cycle of the continuous wave power.

FIG. 6 illustrates the operation of a FM mode-locked CO₂ laser as perthe configuration of FIG. 5(a) with the pulsed RF power applied to thedischarge of the CO₂ laser. The oscilloscope trace has a sweep speed of10 msec per major division, for a burst of mode-locked pulses lasting 50μsec. At the beginning of the laser oscillations, the laser outputradiation was gain switched and a series of higher peak powermode-locked pulse were initiated having a peak power of approximately4.0 times the average power of the steady state mode-locked pulseswithin the RF discharge excitation time. FIG. 7 illustrates theindividual CO₂ laser mode-locked pulses at an oscilloscope sweep speedof 20 nsec per major division. The full-width, half-maximum pulse widthsshown are at 3.76 nsec. The gas pressure was approximately 100 Torr andthe time separation between pulses was approximately 17.8 nsec for themode-locked laser pulses. The crystal orientation for the FM modulatorwas as illustrated in FIG. 5(b). The crystal orientation for Q-switchingand cavity dumping operation and the experimental laser arrangement withthe EO module of FIG. 1, inserted within the CO₂ laser, can be asdescribed in U.S. Pat. No. 6,697,408, incorporated by reference above.

The design of the modulator module illustrated by FIGS. 1-4 can be usedsuccessfully for high duty cycle mode-locking operations, but can bemore complex and costly to manufacture than may be needed for otheroperations, such as for most Q-switching and/or cavity dumpingoperations. For these lower duty cycle operations, there typically willbe less heating of the EO crystal. A simpler EO module design thereforecan be suitable for such operations, which maintains most of theadvantages of the water-cooled embodiment described with respect toFIGS. 1-4.

An embodiment using a simplified design of an EO modulator module 300 isillustrated by the exploded perspective view of FIG. 8, as well as thecross-sectional side view of FIG. 9(a) and the cross-sectional end viewsof FIGS. 9(b) and 9(c). In these Figures, a housing 342 is shown, suchas an aluminum housing, into which fits a lower dielectric holdingstructure 310, such as a BeO holding structure, shaped to hold a lowerelectrode 306, such as a copper electrode. A soft cushion 304, such asan Indium cushion described with respect to FIG. 1, can be placed on theelectrode to gently support the electro-optical crystal 302 as shown inFIG. 9(b), which can be an elongated rectangular CdTe EO crystal asdescribed with respect to FIG. 1. Another soft cushion 304 andelectrically “hot” copper electrode 308 can rest upon the top of the EOcrystal 302, fitting into a recessed area of a top dielectric holdingstructure 312. The Indium cushions 304 again can be used tosubstantially uniformly and firmly hold the EO crystal 302, withoutcausing undue stress on the crystal. BeO again can be chosen for thestructural components due to the exceptional thermal conductivity ofBeO, which is approximately an order of magnitude higher than that of anAlumina ceramic, as well as the relatively low dielectric constant ofBeO, which is approximately ⅓ lower than for an Alumina ceramic. Itshould be recognized that various other materials with similarproperties can be substituted for BeO in fabricating items such as aholding structure 310, 312 in this simplified embodiment.

An electrical connection from the “hot” top electrode 308 to a voltagesource (not shown) such as a high video voltage pulse source can beobtained through use of a copper wire soldered to the hot electrode 308and passed through a top metal plate 314 by an electrical insulatingfitting, such as is shown in the top center of FIG. 9(a). A connectionto the bottom “ground” electrode 306 can be obtained through use of ascrew assembly 328 threaded into the ground electrode 306. The screwassembly can be adjusted to ensure a good electrical ground to the metalhousing 342. A metal flange 326 also can be used to improve the groundconnection. As in FIG. 1, window housings 322 can be used to hold thewindows 324 tight against the EO crystal 302.

Since this modulator design does not utilize liquid cooling, the primarypath for heat removal is through the Indium cushions 304 and copperelectrodes 306, 308 into the BeO holding structures 310, 312, throughthe right side of each BeO holding structure 310, 312 as shown in FIG.9(b) and into the right side of metal housing 342. The heat then can beconducted into a chill plate, for example, used by the Q-switched laser.The housing can include any element(s) known to improve heatdissipation, such as metal fins for increasing the external surface areaof the housing. A spring-loaded screw assembly 334, used to press thecrystal-holding components against the housing in order to assure goodthermal contact, can prevent the components from contacting the leftside of the housing in the Figure. It should be understood that otherdesign choices are possible, such as a tight-fitting housing orinclusion of additional shims or cushions, whereby these componentscontact both sides of the housing for additional heat removal. Since thetop of the upper holding structure 312 is not in contact with the metalcover 314 due to an air space imposed by the top screw assemblies 330,the bottom of the lower BeO holding structure 310 can be designed to notbe in complete contact with the bottom of the metal housing 342, therebyreducing the area through which heat can flow from the bottom of BeOholding structure 310. This provides for substantially equal heatextraction from each of the two surfaces. The edges of the bottom of theholding structure 310 can be pressed against the small surface area ofthe small raised lips 336 along the two inside bottom edges of the metalhousing 342, as shown in FIG. 9(b). This allows a small but equal amountof heat to be removed from the top and bottom ceramic holding structures310, 312.

A small amount of heat also can be removed by the small contact area ofa side BeO holding structure 316 used to apply pressure to a side of theEO crystal, as shown on the left side of FIG. 9(b) applied from theleft-hand side of the metal housing 342. As can be seen, screwassemblies can be used to apply pressure to the holding structure 316 topush the EO crystal to the right in the figure. The EO crystal thenpresses against another side holding structure 318, which in turn ispressed against the wall of the Aluminum housing 342. To reduce the flowof heat from the right (in the figure) side BeO structure 318, so as tomatch the heat flow from the left side BeO structure 316, the surfacecontact area between the housing 342 and the right structure 318 can bereduced to equal the contact area between the side screw assemblies andthe holding structure 316, such as by using a notch 340 or groovemachined in side element 318 as shown in FIG. 8 (and designated by thedotted line area in FIG. 9(b)). An alternate approach would be to insertmetal shims to adjust the amount of contact area in place of a notch orgroove in element 318. A slight depression trench or lip 338 can bemachined near the edges of the upper and lower holding structures 310,312 as shown in FIGS. 8 and 9(b), thereby reducing the heat flow fromthe right structure 318 into the housing 342 to match the heat flow intothe housing 342 (through the upper and lower holding structures) fromthe left side structure 316. An alternate approach can be used to reducethe amount of heat flow from the upper and lower surfaces of the sideholding structures 316, 318 into the lower and upper BeO holdingstructures 310, 312, such as by manufacturing notches 338 in the sidestructures 316, 318 as shown in FIG. 9(c) instead of in the upper andlower holding structures as shown in FIG. 9(b). Either approach canlimit the contact area between these components, thereby reducing theamount of heat flow. Since this design uses top, bottom, and sideholding structures to surround the crystal with a low dielectricconstant material, the capacitance coupling to ground again will belowered such that high video voltage arcing, as well as subsequentdamage to the EO crystal, can be prevented.

A metal cover plate 314 can be bolted or otherwise attached to thehousing 342. Screw assemblies, which also can have associated metalsprings, can pass through the metal cover plate to uniformly push downon the top BeO holding structure 312. A thin rib member 332 can extendfrom the bottom of the cover plate 314, as shown in FIGS. 8 and 9(b),which functions to push the upper BeO holding structure 312 to the rightin FIG. 9(b) and against the metal housing 342 when the cover is inplace.

The thermal design of FIGS. 8-9 provides a sufficient and substantiallyuniform amount of cooling from the top and bottom of the EO crystal 302,through the copper electrodes 306, 308 into the BeO holding structures310, 312 and into the metal housing 342. Due to the screws 334 and theempty space between the housing 342 and internal components 312, 310,316, the thermal design can provide for a smaller but equal amount ofcooling from both sides of the EO crystal 302 into the metal housing342. Since the cooling from the sides is somewhat lower in thisembodiment, the EO crystal 302 can experience a small refractive indexvariation in the vertical direction, which can lead to a cylindricallens effect in the vertical direction of FIG. 9(b) or FIG. 9(c), sincethe center of the crystal is hotter than the ends. More importantly,however, this cooling approach maintains constant cooling along thehorizontal axis, thereby minimizing minimize the refractive indexvariation along the horizontal direction of the crystal due to the smallbut equal amount of heat that flows horizontally in the Figure. Themaintaining of a substantially constant temperature along the horizontalaxis of the EO crystal can result in a lack of a temperature gradientsuch that there is little to no radiation deflection in the horizontaldirection. The small lens effect experienced in the vertical directioncan be tolerated by a Q-switched laser, and typically will notappreciably alter performance. On the other hand, the little to noradiation deflection existing in this Q-switched modulator designgreatly improves the performance of a Q-switched CO₂ waveguide laserbeam.

FIG. 10 illustrates as a function of time, with 1 minute per division,the power output of a Q-switched waveguide CO₂ laser utilizing amodulator design of the prior art. The laser used to gather the data ofFIG. 10 utilized a modulator design as shown and described with respectto FIGS. 20 and 21 in U.S. Pat. No. 6,697,408 B2, incorporated byreference above, which includes both beam deflection and lensingconditions. The power reached a steady state value of approximately 46Wafter approximately 2 minutes of warm-up time. FIG. 10 also illustratesas a function of time the far field output beam divergence variations,in microradians, along both the vertical (Y) and horizontal (X) axes. Itcan be seen that quickly after the start, the output beam divergence inboth the X and Y axes raises to greater than 10,000 microradians. Inapproximately 0.2 minutes the beam divergence drops to approximately6,300 μrad for the vertical (Y) and 3,400 μrad for the horizontal (X)axis. The difference of approximately two times between the divergenceof the horizontal and vertical beam directions is due to the fact thatthe waveguide channel is rectangular in shape with the vertical height(0.11 inches) being ½ of the horizontal width (0.22 inches).

FIG. 11 illustrates the behavior of the same laser with a Q-switchingmodulator module using a design as described with respect to FIGS. 8-9.It can be seen that the power output rises to the steady state power ofapproximately 48W in about 0.1 minute or less. This corresponds to animprovement of about 20 times over the prior art modulator design. Italso can be seen that the far field beam divergence raises to 6,880 μradand 3,900 μrad for the Y and X axes, respectively. The far field beamdivergence then drops to the steady state value in approximately 0.2minute. Clearly, the beam divergence results of FIG. 11 are superior tothe results of FIG. 10.

It should be recognized that a number of variations of theabove-identified embodiments will be obvious to one of ordinary skill inthe art in view of the foregoing description. Accordingly, the inventionis not to be limited by those specific embodiments and methods of thepresent invention shown and described herein. Rather, the scope of theinvention is to be defined by the following claims and theirequivalents.

1. An electro-optical modulator module for use in a CO₂ laser system, comprising: an active optical crystal having an optical entrance end surface and an optical exit end surface; a pair of wall structures positioned on opposing sides of the active optical crystal and being in thermal contact with the optical crystal; and at least one cooling passage in thermal contact with each of the pair of wall structures, each cooling passage capable of containing a flow of fluid for removing heat transferred from the crystal.
 2. A module according to claim 1, wherein: the active optical crystal is a CdTe crystal.
 3. A module according to claim 1, wherein: the active optical crystal is an elongated rectangular crystal.
 4. A module according to claim 1, wherein: each of the pair of wall structures is formed of a Beryllium Oxide (BeO) ceramic material.
 5. A module according to claim 1, wherein: each of the pair of wall structures is formed of a low dielectric constant material capable of preventing arcing across the active optical crystal.
 6. A module according to claim 1, further comprising: a pair of electrodes on opposing sides of the active optical crystal and capable of applying an electric field across the active optical crystal, where each of the pair of wall structures is positioned to press one of the pair of electrodes against a respective side of the crystal, whereby heat from the crystal passes through the electrodes to the pair of wall structures.
 7. A module according to claim 6, further comprising: a metal cushion positioned between each of the pair of the electrodes and the active optical crystal, each metal cushion capable of ensuring thermal and electrical contact between the electrodes and the crystal without placing undue stress on the crystal, each metal cushion allowing for a normal expansion and contraction of the crystal.
 8. A module according to claim 7, wherein: each metal cushion is an Indium metal cushion.
 9. A module according to claim 1, wherein: each wall structure contains two parallel cooling passages positioned symmetrically with respect to the active optical crystal.
 10. A module according to claim 9, wherein: the two parallel cooling passages allow a fluid to flow in opposing directions.
 11. A module according to claim 1, wherein: the fluid flowing through the cooling passages is a mixture including at least one corrosion inhibitor.
 12. A module according to claim 1, further comprising: a pair of ceramic holders positioned on opposing sides of the active optical crystal and in contact with at least one of the wall structures, the pair of ceramic holders positioned such that heat can flow from the crystal into the pair of ceramic holders in a direction that is substantially orthogonal to the direction in which heat flows into the wall structures.
 13. A module according to claim 12, wherein: the pair of wall structures and pair of ceramic holders provide for symmetric cooling along two axes of the crystal.
 14. A module according to claim 1, wherein: the module can be used for at least one of Frequency Modulator (FM) and Amplitude Modulator (AM) mode-locking.
 15. A module according to claim 1, further comprising: a metal housing for containing the active optical crystal and pair of wall structures, the metal housing providing RF shielding for mode-locking operation.
 16. A module according to claim 1, further comprising: a pair of window housings positioned to hold a window against each end surface of the active optical crystal.
 17. A module according to claim 1, wherein: the at least one cooling passage is disposed within each of the pair of wall structures.
 18. A method of symmetrically cooling an active optical crystal in a modulator module of a CO₂ laser system, comprising the steps of: positioning a pair of wall structures on opposing sides of the active optical crystal such that the wall structures are in thermal contact with the optical crystal, each wall structure having at least one cooling passage in thermal contact therewith; and directing a flow of fluid through each cooling passage in order to remove heat transferred from the active optical crystal.
 19. A method according to claim 18, further comprising: forming each of the pair of wall structures of a low dielectric constant material capable of preventing arcing across the active optical crystal.
 20. A method according to claim 18, further comprising: positioning a pair of electrodes on opposing sides of the active optical crystal such that each of the pair of wall structures presses one of the pair of electrodes against a respective side of the crystal, the pair of electrodes capable of applying an electric field across the active optical crystal and passing heat from the active optical crystal to the wall structures.
 21. A method according to claim 20, further comprising: positioning a metal cushion between each of the pair of the electrodes and the active optical crystal, each metal cushion capable of ensuring thermal and electrical contact between the electrodes and the crystal without placing undue stress on the crystal, each metal cushion allowing for a normal expansion and contraction of the crystal.
 22. A method according to claim 18, further comprising: forming at least two parallel cooling passages in each wall structure, the parallel cooling passages positioned symmetrically with respect to the active optical crystal.
 23. A method according to claim 22, further comprising: passing fluid through the at least two parallel cooling passages in opposing directions.
 24. A method according to claim 18, further comprising: positioning a pair of ceramic holders on opposing sides of the active optical crystal and in contact with at least one of the wall structures, the pair of ceramic holders positioned such that heat can flow from the crystal into the pair of ceramic holders in a direction that is substantially orthogonal to the direction in which heat flows into the wall structures.
 25. An electro-optical modulator module for use in a CO₂ laser system, comprising: an active optical crystal having an optical entrance end surface and an optical exit end surface; a pair of wall structures positioned on opposing sides of the active optical crystal and being in thermal contact with the optical crystal, whereby heat from the crystal can pass symmetrically into the pair of wall structures; and a pair of ceramic holders positioned on opposing sides of the active optical crystal and in contact with at least one of the pair of wall structures, the pair of ceramic holders positioned such that heat can flow from the crystal into the pair of ceramic holders in a direction that is substantially orthogonal to the direction in which heat flows into the wall structures.
 26. A module according to claim 25, wherein: the pair of wall structures and pair of ceramic holders provide for symmetric cooling along two axes of the crystal.
 27. A module according to claim 25, wherein: heat flowing into the pair of ceramic holders flows into the at least one of the pair of wall structures in contact with the pair of ceramic holders.
 28. A module according to claim 25, wherein: the active optical crystal is a CdTe crystal.
 29. A module according to claim 25, wherein: the active optical crystal is an elongated rectangular crystal.
 30. A module according to claim 25, wherein: each of the pair of wall structures is formed of a Beryllium Oxide (BeO) ceramic material.
 31. A module according to claim 25, wherein: each of the pair of wall structures is formed of a low dielectric constant material capable of preventing arcing across the active optical crystal.
 32. A module according to claim 25, further comprising: a pair of electrodes on opposing sides of the active optical crystal and capable of applying an electric field across the active optical crystal, where each of the pair of wall structures is positioned to press one of the pair of electrodes against a respective side of the crystal, whereby heat from the crystal passes through the electrodes to the pair of wall structures.
 33. A module according to claim 32, further comprising: a metal cushion positioned between each of the pair of the electrodes and the active optical crystal, each metal cushion capable of ensuring thermal and electrical contact between the electrodes and the crystal without placing undue stress on the crystal, each metal cushion allowing for a normal expansion and contraction of the crystal.
 34. A module according to claim 33, wherein: each metal cushion is an Indium metal cushion.
 35. A module according to claim 25, further comprising: a metal housing for containing the active optical crystal, pair of ceramic holders, and pair of wall structures, the metal housing providing RF shielding for mode-locking operation.
 36. A module according to claim 25, further comprising: a pair of window housings positioned to hold a window against each end surface of the active optical crystal.
 37. A method of symmetrically cooling an active optical crystal in a modulator module of a CO₂ laser system, comprising the steps of: positioning a pair of wall structures on opposing sides of the active optical crystal such that the wall structures are in thermal contact with the optical crystal, the walls capable of receiving heat passed from the active optical crystal; and positioning a pair of ceramic holders on opposing sides of the active optical crystal and in contact with at least one of the pair of wall structures, the pair of ceramic holders positioned such that heat can flow from the crystal into the pair of ceramic holders in a direction that is substantially orthogonal to the direction in which heat flows into the wall structures.
 38. A method according to claim 37, wherein: positioning the pair of ceramic holders in contact with at least one of the pair of wall structures allows heat flowing into the pair of ceramic holders to subsequently flow into the at least one of the pair of wall structures in contact with the pair of ceramic holders.
 39. A method according to claim 38, further comprising: forming each of the pair of wall structures of a low dielectric constant material capable of preventing arcing across the active optical crystal.
 40. A method according to claim 38, further comprising: positioning a pair of electrodes on opposing sides of the active optical crystal such that each of the pair of wall structures presses one of the pair of electrodes against a respective side of the crystal, the pair of electrodes capable of applying an electric field across the active optical crystal and passing heat from the active optical crystal to the wall structures.
 41. A method according to claim 40, further comprising: positioning a metal cushion between each of the pair of the electrodes and the active optical crystal, each metal cushion capable of ensuring thermal and electrical contact between the electrodes and the crystal without placing undue stress on the crystal, each metal cushion allowing for a normal expansion and contraction of the crystal.
 42. An electro-optical modulator module for use in a CO₂ laser system, comprising: an elongated active optical crystal having an optical entrance end surface and an optical exit end surface, the elongated active optical crystal being rectangular in cross-section; a pair of dielectric holding structures positioned on opposing sides of the rectangular active optical crystal and being in thermal contact with the optical crystal, whereby heat from the crystal can pass symmetrically into the pair of dielectric holding structures; and a pair of dielectric side structures positioned on opposing sides of the active optical crystal and each in contact with the pair of dielectric holding structures, such that each of the sides of the rectangular crystal are substantially in contact with one of the pair of dielectric holding structures or the pair of dielectric side structures, the pair of dielectric side structures positioned such that heat can pass symmetrically into the pair of dielectric side structures in a direction that is substantially orthogonal to the direction in which heat flows into the pair of dielectric holding structures.
 43. A module according to claim 42, wherein: heat flowing into the pair of dielectric side structures subsequently flows into the pair of dielectric holding structures.
 44. A module according to claim 42 wherein: the active optical crystal is a CdTe crystal.
 45. A module according to claim 42, wherein: each of the pair of dielectric holding structures and each of the pair of dielectric side structures is formed of a Beryllium Oxide (BeO) ceramic material.
 46. A module according to claim 42, wherein: each of the pair of dielectric holding structures and each of the pair of dielectric side structures is formed of a low dielectric constant material capable of preventing arcing across the active optical crystal.
 47. A module according to claim 42, further comprising: a pair of electrodes on opposing sides of the active optical crystal and capable of applying an electric field across the active optical crystal, where each of the pair of dielectric holding structures is positioned to press one of the pair of electrodes against a respective side of the crystal, whereby heat from the crystal passes through the electrodes to the pair of dielectric holding structures.
 48. A module according to claim 47, further comprising: a metal cushion positioned between each of the pair of the electrodes and the active optical crystal, each metal cushion capable of ensuring thermal and electrical contact between the electrodes and the crystal without placing undue stress on the crystal, each metal cushion allowing for a normal expansion and contraction of the crystal.
 49. A module according to claim 48, wherein: each metal cushion is an Indium metal cushion.
 50. A module according to claim 42, further comprising: a metal housing for containing the active optical crystal, pair of dielectric holding structures, and pair of dielectric side structures.
 51. A module according to claim 42, further comprising: a pair of window housings positioned to hold a window against each end surface of the active optical crystal.
 52. A method of symmetrically cooling an elongated, rectangular active optical crystal in a modulator module of a CO₂ laser system, comprising the steps of: positioning a pair of dielectric holding structures on opposing sides of the rectangular active optical crystal and in thermal contact with the optical crystal, whereby heat from the crystal can pass symmetrically into the pair of dielectric holding structures; and positioning a pair of dielectric side structures on opposing sides of the active optical crystal and each in contact with the pair of dielectric holding structures, such that each of the sides of the rectangular crystal are substantially in contact with one of the pair of dielectric holding structures or the pair of dielectric side structures, the pair of dielectric side structures positioned such that heat can pass symmetrically into the pair of dielectric side structures in a direction that is substantially orthogonal to the direction in which heat flows into the pair of dielectric holding structures.
 53. A method according to claim 52, further comprising: forming the pair of dielectric holding structures and the pair of dielectric side structures of a low dielectric constant material capable of preventing arcing across the active optical crystal.
 54. A method according to claim 52, further comprising: positioning a pair of electrodes on opposing sides of the active optical crystal such that each of the pair of dielectric holding structures presses one of the pair of electrodes against a respective side of the crystal, the pair of electrodes capable of applying an electric field across the active optical crystal and passing heat from the active optical crystal to the dielectric holding structures.
 55. A method according to claim 54 further comprising: positioning a metal cushion between each of the pair of the electrodes and the active optical crystal, each metal cushion capable of ensuring thermal and electrical contact between the electrodes and the crystal without placing undue stress on the crystal, each metal cushion allowing for a normal expansion and contraction of the crystal. 